The aspects of the disclosed embodiments are in the field of structural parts made of a composite material with reinforced fibers. More specifically, the aspects of the disclosed embodiments concern a curved structural part used, for example, to make a circumferential stiffener or frame of a shell structure, such as an aircraft fuselage. The aspects of the disclosed embodiments also concern a process for manufacturing such a part and a device for implementing such a process.
In the prior art, such a structural part, FIG. 1, comes in a form whose approximately constant cross section (1) can generally be L, U, Z or J-shaped or more complex forms, with said cross section extending along a non-straight scanning axis (2). The scanning axis (2) is a curve characterized by a succession of radii of curvature (20), FIG. 2, combined at their respective center of curvature (21). The cross section of such a profile includes at least two wings, at least one of them (10) further comprising a radius of curvature that varies with its width (201, 202).
Such structural parts, when they are composed of a composite reinforced-fiber material, are obtained from fibers stacked in a certain direction and in a definite sequence. An example of such a profile is described in French patent application FR20070057431 in the name of the applicant.
In a first embodiment, said profile is obtained by placing a plurality of layers of dry fibers, or plies, with a definite orientation in a mold that has the shape of the section and the curve of the part. Then, the fibers are immersed in the resin by resin transfer or infusion.
Alternately, such structural parts can be obtained by draping plies of impregnated fibers followed by consolidation and curing. Patent EP0436415 describes a process for obtaining such a structural part in which a stack of dry or pre-impregnated fibers, straight and flat, is first made, then applied to a tool with the desired cross section and curve by means of a bladder. The plies initially have an orientation defined in relation to the longitudinal axis of the straight stack. This orientation is kept after the strip is applied to the tools when it is measured in relation to the curved axis of the local curve or, along a complementary angle, in relation to the local radius of curvature.
Patent application WO 2005/011961 describes a process for obtaining such a part in the prior art in which the strips of pre-impregnated material are applied directly to the sectioning and curving tools adapted by means of a draping head combined with pressure rollers.
This operation, which consists of pressing and tightening a fiber preform composed of continuous fibers, pre-impregnated or not, on a curved tool is traditionally called “spreading.” For aeronautical applications, the fibers are commonly composed of carbon, and the matrix of a thermosetting resin.
Such fibers have no capacity for plastic deformation so it is difficult, or even impossible, to spread a strip or nap of fibers whose orientation is perpendicular to the local radius of curvature of the form, in the 0° direction, FIG. 3, and whose radius of curvature varies with the width of the strip. Trying to spread such a nap leads to the formation of puckers in said nap, puckers that are particularly damaging to the mechanical hold of the part that contains such a nap of fibers. The difficulty with spreading will be even greater the closer the orientation of fibers placed this way is to the curve, i.e., the angle of orientation of the fibers α, FIG. 3, is close to 0°, the wider the nap and the smaller the radius of curvature, i.e., the higher the gradient of curvature in the width of the nap and the greater the length of the nap.
In effect, when the nap fibers are oriented at an angle α, close to 0°, the spreading is produced by sliding between fibers, parallel to the curve; said sliding must be produced in the resin over the entire length of the fibers. Consequently, it is very hard to place the nap gradually while ensuring such sliding.
When the fibers are not perpendicular to the local radius of curvature, and the radius of curvature of the strip varies according to its width, the spreading is produced by a gradual modification, correlative to the radius of curvature, in the space between the fibers (3), FIG. 4.
In this embodiment, consider, for example, a part whose cross section is Z-shaped, FIG. 1; the part of the fibers constituting the preform is in the wing (12) which has no variation in the radius of curvature according to its width and located on the minimum radius of curvature will have no variation in the spacing of the fibers after it is formed. In fact, this part does not undergo spreading and can therefore contain fibers oriented perpendicular to the radius of curvature, that is, parallel to the scanning axis. The center wing (10), currently called the “core”, has a variation in curvature according to its width. The portion of fibers located in the core undergoes spreading, and after the fibers are applied to the form, they are spaced more on the largest side of the radius of curvature. The variation in spacing is such that it is proportional to the variation in the radius of curvature and maintains the nominal orientation of the fibers with regard to the curved local axis of curvature. The part of the fibers located in the exterior wing (11) corresponding to the maximum radius of curvature, but which has no variation in the radius of curvature over its width undergoes spreading, but with no variation in the spacing of the fibers over its width.
The spreading of such a section therefore requires precautions in terms of the exterior wing (11), since the parts of fiber located in this portion must first undergo spreading, leading to a variation in the spacing of the fibers proportional to the variation in the radius of curvature, then a tightening of said fibers when the corresponding part is applied to the part of the form forming the wing. To keep it from puckering on this occasion, all of the fibers applied to the form must always be kept under tension. This characteristic is obtained in EP0436415 by the method of progressive action of the bladder and in 202005/011961 by the gradual application, along the curved abscissa, of the fiber preform on the tool, by the pressure rollers.
Such a structural part obtained in this way can be used advantageously to make a shell structure, such as an aircraft fuselage. Making the part of a composite reinforced-fiber material gives it a lower weight than a part made of a metal material with equal mechanical resistance.
Considering the large size of aircraft fuselages for commercial transport, to make them easier to assemble, a structural element such as a frame is composed of several structural pieces forming sectors on the circumference of said fuselage. These sectors are assembled by splicing. Moreover, the interior floor is also connected to the frames, so that the weight of the floor, the passengers and the commercial load that it supports is drained and distributed in the shell structure of which the fuselage is composed. All these connections are generally made with rivet-type fasteners. Such a connection must have enough fasteners to transfer the mechanical load that it supports from one piece to another. And, for it to resist fatigue and peening, rules of spacing between the fasteners must be followed. Now, when the frames are made of a composite reinforced-fiber material, the section of the parts necessary to absorb the service forces does not make it possible to install the number of fasteners suited to take the different loads at the connections while following the spacing rules for said fasteners. One solution consists of using a profile with a wider core to be able to space the fasteners. This solution, besides increasing the weight of the parts, reduces the interior space in the cabin, and hence the volume available for the commercial load in the aircraft. The solutions in the prior art consist of interspersing in these connections, complex splicing parts, generally made of metal. However, such solutions come at a high cost, are detrimental in terms of weight and, due to the high rigidity of the connecting pieces, result in the transmission of parasitic stresses into the structures.
There is therefore a need for composite structural elements, such as circumferential stiffeners of the aircraft fuselage, or frames, whose various connections can be made without such complex parts, but without the widening of the profile being detrimental to the volume available for the commercial load within the shell structure supported by those frames.